Mechanistic study of hafnium and zirconium extraction with organophosphorus extractants

Research on the purification of Zr has increased in the last few decades and although effective processes have been developed, little is known about the mechanism of Hf and Zr extraction; particularly, at low acid concentrations. This study’s aim was to determine the influence of alterations in alkyl substituents on the extraction mechanism of Hf and Zr from sulfate media. Seven organophosphorus extractants (Dio-PA, TPPA, DPPA, TPPO, TPPS, D2EHPA and Ionquest 801) were screened with four of them (Dio-PA, DPPA, D2EHPA and Ionquest 801) resulting in promising extraction results. This was attributed to the activity of the acidic hydroxy phosphinyl (=P(O)OH) group which is responsible for the dimerization and cation exchange properties of the extractant. The extractant and hydrogen ion stoichiometries involved in the extraction mechanism were determined for these four extractants by slope analysis from extractant- and acid-variation experiments. It was found that Dio-PA, D2EHPA, and Ionquest 801 extract via a cation-exchange mechanism, according to [M(OH)2]2+ + 2 HA ⇌ M(OH)2A2 + 2 H+. In contrast, the extraction with DPPA was best described by a solvated cation exchange (SCE) mechanism, according to [M(OH)]3+ + 6 HA ⇌ M(OH)A3.3HA + 3 H+. It was concluded that the presence of electron-withdrawing substituents produced softer Lewis base extractants, which favored the extraction of Hf over Zr from acidic sulfate media.